The present invention relates to a ceramic body and a process for making the same. In particular, the present invention relates to a ceramic body formed of magnesia-partially-stabilized zirconia (Mg-PSZ) material having a unique combination of high strength, wear resistance, and corrosion resistance.
The known Mg-PSZ materials are composed of zirconia with about 2.5-3.5% by weight magnesia, heated to form a single phase cubic zirconia, and then cooled in a controlled manner to develop a dispersion of tetragonal zirconia precipitates within the cubic grains. Much of the strength and toughness provided by the known Mg-PSZ materials results from the potential for stress induced martensitic transformation of the tetragonal precipitates to the monoclinic crystalline form when the material is stressed under load. Associated with the martensitic transformation is a volume expansion of those precipitates. When the material is placed under load, transformation occurs locally at sites of weakness, such as pre-existing cracks, pores, or other voids within the material which experience high stress intensity. The localized transformation zones reduce the stress levels experienced at the sites of weakness and thereby increase the strength of the material as a whole.
Despite this strengthening mechanism, one factor limiting the strength of the known Mg-PSZ materials is the large cubic zirconia grain size of those materials, typically between about 50 and 100 microns in major dimension. The cubic zirconia grains become enlarged because of the high processing temperatures used to achieve the single cubic solid solution phase from which the final microstructure is developed. The high processing temperatures result in high diffusion rates and, hence, rapid grain growth of the cubic phase.
Associated with the large cubic zirconia grains are large pores, voids, and other intrinsic microstructural flaws which limit the strength of the known Mg-PSZ materials. As a result, the known Mg-PSZ materials have been restricted in their use. For example, the known Mg-PSZ materials are unsuitable for some guide, bushing, and forming tool applications in the metals processing industry or for severe service valve components because the known materials are unable to provide the requisite combination of high strength, wear resistance, and corrosion resistance.
In light of the foregoing, it would be highly beneficial to provide a Mg-PSZ material, and articles formed therefrom, that have a significantly higher strength than the known Mg-PSZ materials. Further, the material should provide the high strength without adversely effecting the wear resistance and corrosion resistance of the material.
The property limitations associated with the known Mg-PSZ materials are overcome, to a large extent, by a ceramic body in accordance with the present invention. The present invention provides a ceramic body made from a Mg-PSZ material, and a process for producing the same, wherein a small volume fraction of a grain growth inhibiting material is added to a Mg-PSZ material in order to limit the grain boundary movement of the cubic zirconia phase. The volume fraction and particle size of the grain growth inhibiting material, as well as the method of introduction and heat treatment, are controlled to limit grain growth and thus provide higher strength than the known Mg-PSZ materials. More specifically, a ceramic body produced in accordance with the present invention provides a flexural strength that is significantly higher than that provided by the known Mg-PSZ materials.
In accordance with one aspect of the present invention, there is provided a ceramic body made from a ceramic powder composition containing about 2.8-5.0% by weight magnesia, an effective amount of a grain growth inhibiting material, and the balance essentially zirconia. The grain growth inhibiting material is selected to be insoluble and stable in the Mg-PSZ system at the high temperatures used during processing (1600xc2x0 C. to 1850xc2x0 C). Examples of suitable grain growth inhibiting materials are magnesium aluminate spinel (MgAl2O4), silicon carbide (SiC), and titanium carbide (TiC). Nitrides, borides, and other types of carbides should also be suitable as grain growth inhibiting agents.
The microstructure of the ceramic body comprises grains of cubic zirconia having an average grain size of less than about 30 microns in major dimension and 0.1-8.7% by volume of discrete particles of the grain growth inhibiting material. Preferably, the discrete particles of grain growth inhibiting material have an average size less than about 5 microns in major dimension and reside primarily at the grain boundaries of the cubic zirconia. However, a portion of the discrete particles may reside within the grains of cubic zirconia. In one embodiment, the grain growth inhibiting material comprises magnesium aluminate spinel (MgAl2O4).
Additionally, discrete precipitates of tetragonal zirconia are distributed within the grains of cubic zirconia. Preferably, the tetragonal precipitates have a substantially ellipsoidal shape with a long dimension of about 0.1-0.4 microns.
In another aspect, the present invention relates to a method of making a ceramic body. The method comprises the step of mixing a zirconia powder, a magnesia powder, and a grain growth inhibiting additive to form a homogeneous powder mixture. Alternatively, some or all of the magnesia powder can be replaced with a magnesium salt which decomposes upon heat treatment to form magnesia. In one embodiment, the additive powder comprises magnesium aluminate spinel. In an alternate embodiment, the additive powder comprises alumina which can react with some of the magnesia during subsequent processing to form magnesium aluminate spinel particles.
The homogeneous powder mixture is consolidated into a green body which is then fired in air to a temperature in the range of about 1600xc2x0 C. to about 1850xc2x0 C. to develop a substantially fully dense sintered body. When carbides, nitrides, or borides are used as the grain growth inhibiting agent, the firing is conducted in a nonoxidizing or reducing atmosphere.
During the firing step, the grain growth inhibiting particles restrain (pin) the movement of the zirconia grain boundaries. The firing step is conducted to a temperature and for a time sufficient to ensure that all, or nearly all, of the zirconia is in the cubic crystalline form without producing cubic grains having excessively large sizes.
The sintered body is then cooled at a controlled cooling rate to nucleate and grow tetragonal zirconia precipitates within the cubic zirconia grains of the body. In one embodiment, the sintered body is cooled from the maximum firing temperature to about 1400xc2x0 C. at a rate in the range of about 100xc2x0 C./hr to about 500xc2x0 C./hr; from about 1400xc2x0 C. to about 1200xc2x0 C. at a rate in the range of about 40xc2x0 C./hr to about 200xc2x0 C./hr; and from about 1200xc2x0 C. to below about 600xc2x0 C. at a rate in the range of about 100xc2x0 C./hr to about 300xc2x0 C./hr. Once cooled, the ceramic body may be optionally heat treated at a temperature in the range of about 1000xc2x0 C. to about 1500xc2x0 C. to further develop its microstructure.